1. Field of the Invention
The present invention is directed to molding compositions and molded bodies prepared from these compositions which are used to form molded plastic parts. More particularly, the invention is directed to molding compositions containing an unsaturated polyester, a mixture of styrene and a di(meth)acrylic acid ester of a (cyclo)aliphatic diol, natural fibers and additives.
2. Discussion of the Background
Molding compositions prepared from unsaturated polyesters, styrene, natural fibers and additives and molded bodies obtained therefrom via hot pressing, i.e. molded parts comprised of natural fiber reinforced unsaturated polyester (UP) resins, have been known for a long time. For years the automobile industry has employed jute fiber reinforced molded parts comprised of UP resins combined with jute mats (with the ratio of jute to resin usually being 1:1 by weight), manufactured by hot pressing. These parts are further processed to form automobile interior surface parts. The hardened material obtained in the hot pressing process is a valuable construction material.
Natural fibers such as jute, sisal, and flax have high ability to absorb water. Therefore they must be dried before processing. After the drying there is some residual water content, the amount of which may vary. This water inhibits the radical polymerization of styrene which occurs during hot pressing. In practice, jute fiber containing molded parts have a residual styrene content of as much as 0.4-1.2 wt. %. This gives them a more or less strong odor of styrene. A number of studies have been undertaken to attempt to eliminate this disadvantage. Process modifications such as variation of the UP resins, of the polymerization initiators for the radical polymerization, and of the temperature schedule during the hot pressing and the careful drying of the jute mats, have not enabled reliable manufacture of molded parts having a residual styrene content of &lt;0.5 wt. %.
Intensified environmental concern and the drastic reduction of the MAC (maximum allowable concentration) for styrene in 1987 from 100 pp to 20 ppm, have led to the requirement placed on molded parts manufacturers by the automobile manufacturers that the parts manufacturers produce jute reinforced molded parts having a residual styrene content of &lt;0.1 wt. %.
Studies aimed at complete or partial replacement of the styrene in the molding compounds with diallyl phthalate have not led to the desired result. Firstly, the viscosity of the resins is too high, making it impossible to adequately impregnate the jute mats. Further, because the reaction is relatively slow, the residence times in the hot pressing process are too long. Secondly a representative resin composition of 52 parts by weight (pbw) of an unsaturated polyester, 20 pbw styrene, and 28 pbw diallyl phthalate will give a sufficiently low resin viscosity, a sufficiently short residence time in the hot pressing, good surface quality, and good mechanical strength parameters of the molded parts, as well as low (&lt;0.1%) residual styrene content in the molded parts. However, there is a residual content of diallyl phthalate of 4-8 wt. % in the molded parts, as determined by gas chromatographic analysis. Even with the use of the best polymerization initiators, the residual monomer content is not appreciably reduced. The molded parts are unsuitable for the major intended applications, because the automotive interior surface elements produced from such parts would lead to "fogging", i.e. formation of a coating on the interior surfaces of the windowpanes of the passenger compartment.
There is a continuing need for natural fiber containing molding compounds based on UP resins, which overcome the above-described disadvantages, wherewith molded parts can reliably be obtained from the molding compounds by not pressing, which parts have a residual styrene content of &lt;0.1 wt. %.